The present invention relates to a driving device and driving method of a liquid crystal display device for improving display quality of a liquid crystal display device of a simple matrix type.
In a conventional liquid crystal display device of a simple matrix type, as shown in FIG. 10, a segment side driving circuit (segment driver) 72 and a common side driving circuit (common driver) 73 are connected to a liquid crystal panel 71. To the segment side driving circuit 72 and the common side driving circuit 73 are connected a power source circuit 74 and a controller 75. The power source circuit 74 supplies power to the segment side driving circuit 72 and the common side driving circuit 73. The controller 75 sends various control signals to the segment side driving circuit 72 and the common side driving circuit 73.
The controller 75 gives the segment side driving circuit 72 display data, a data latch clock for taking in the display data, a horizontal synchronizing signal, and an alternating signal for driving the liquid crystal panel 71 in AC. The controller 75 gives the common side driving circuit 73 a horizontal synchronizing signal, an alternating signal, and a vertical synchronizing signal for recognizing a start of a screen.
The power source circuit 74 supplies voltages of VBH and VBL to the segment side driving circuit 72, and voltages of VH, VC, and VL to the common side driving circuit 73. The liquid crystal panel 71 constitutes a simple matrix arrangement by segment electrodes X1, X2, X3, . . . , and Xm and common electrodes Y1, Y2, Y3, . . . , and Yn, and a liquid crystal cell 76 constitutes a pixel at the intersection of the segment electrodes and the common electrodes.
FIG. 11 shows an internal structure of the conventional segment side driving circuit 72. The segment side driving circuit 72 includes a shift register 77, a data latch 78, a line latch 79, a level shifter 80, and a liquid crystal driving output circuit 81. In the case where the segment side driving circuit 72 has, for example, 240 outputs, the display data of 240 lines are serially inputted to the shift register 77 in synchronization with the data latch clock. The display data then are converted into parallel data in the shift register 77, and are latched in the data latch 78 and accumulated therein.
When data of the number of column electrodes of the liquid crystal panel 71 are accumulated, the horizontal synchronizing signal (LP) is inputted, and the display data accumulated are latched by the line latch 79. The display data then are converted by the level shifter 80 from a logic voltage level to a liquid crystal driving voltage level to be inputted to the liquid crystal driving output circuit 81. The liquid crystal driving output circuit 81 outputs a driving output voltage to the segment electrodes X1, . . . , and X240 of the liquid crystal panel 71 in accordance with the display data inputted from the level shifter 80 and in accordance with the alternating signal (FR).
FIG. 12 shows operations of the prior art. For convenience, the explanation will be given through the case where the number of common electrodes Y (scanning electrodes) is, for example, seven, that is, the case where the alternating signal is switched per seven scanning periods.
A common output voltage VY applied to the common electrodes Y as scanning electrodes receives the vertical synchronizing signal and outputs, in accordance with the horizontal synchronizing signal and the alternating signal from a head line, either VH level or VL level when the electrodes are selected, and outputs VC level when the electrodes are not selected (non-select period). A segment output voltage VX applied to the segment electrodes X as data electrodes is selected as one of VBH and VBL levels in accordance with the display data and the alternating signal, and the whole output of a single scanning electrode is applied in parallel to the segment electrodes X.
In the liquid crystal panel 71, a potential difference between the segment electrodes X and the common electrodes Y is applied to each pixel, and display or non-display is decided in accordance with an effective value of the potential difference in a single frame period, which is the time required to display one screen. In the prior art, the display color or gradation on the liquid crystal panel 71 is slightly different depending on display data or display pattern even by comparison within the same ON display or OFF display.
The following describes one example of luminous non-uniformity generated by display data.
{circle around (1)} As shown in FIG. 13, in the case of a whole screen ON display, the segment output voltage from each segment side driving circuit 72 is maintained at a constant voltage level unless the alternating signal is changed.
The effective voltage VeffA of voltage VA applied to the liquid crystal cell 76 as a pixel of the liquid crystal panel 71 is represented by a difference between a potential of a segment output waveform and a potential of a common output waveform.
{circle around (2)} As shown in FIG. 14, in the case of a stripe display in which display data repeats ON and OFF per scanning line, the segment output voltage from each segment side driving circuit 72 is outputted with alternating and repeating VBH and VBL levels per fall of the horizontal synchronizing signal even when the alternating signal is not changed.
The effective voltage VeffB of voltage VB applied to the liquid crystal cell 76 as a pixel of the liquid crystal panel 71 is represented by a difference between a potential of a segment output waveform and a potential of a common output waveform.
The difference between {circle around (1)} and {circle around (2)} is that the number of changes of the output level of the segment output waveform per unit time is different. When the output level changes, the output waveform is blunted by such factors as the capacitance of the liquid crystal cell 76, the electrode resistance of the electrodes of the liquid crystal panel 71, and the output resistance of the driving circuits 72 and 73, and therefore when the number of changes of the output level is large, the effective voltage is reduced by blunted waveform.
Thus, even when there are the same numbers of ON display pixels and OFF display pixels, the effective voltage VeffB, in which the number of changes of the output level of the segment output waveform is made larger, satisfies the relationship of VeffB  less than VeffA relative to the effective voltage VeffA. As a result, by this reduction in effective voltage, luminous non-uniformity called shadowing is generated on the display screen.
The following describes an example of luminous non-uniformity generated when the alternating signal is switched.
{circle around (3)} As shown in FIG. 15, in the case of a whole screen ON display in which the alternating signal is switched at least once in a single frame period, the segment output voltage from the segment side driving circuit 72 is outputted with alternating and repeating VBH and VBL levels in accordance with the alternating signal. When the alternating signal is switched, all the segment output voltages are switched at once from (a) VBH to VBL or from (b) VBL to VBH. Here, distortion due to a voltage shift is generated on the common electrodes Y, which are scanning electrodes facing the segment electrodes X with the liquid crystal therebetween, in accordance with the ratio of the change [(a) and (b)] in the segment electrodes X.
The effective voltage VeffC of voltage VC applied to the liquid crystal cell 76 which is a pixel of the liquid crystal panel 71 is represented by a difference between a potential of the segment output waveform and a potential of the common output waveform.
{circle around (4)} As shown in FIG. 16, in the case of a partially OFF block display, as with the case {circle around (3)}, the segment output voltages change all at once, and thus there is generated distortion due to a voltage shift on the common electrodes Y, which are scanning electrodes facing the segment electrodes X with the liquid crystal therebetween, in accordance with the ratio of the change [(a) and (b)] of the segment electrodes X.
The effective voltage VeffD of voltage VD applied to the liquid crystal cell 76 which is a pixel of the liquid crystal panel 71 is represented by a difference between a potential of the segment output waveform and a potential of a common output waveform.
The difference between {circle around (3)} and {circle around (4)} is that the directions of distortion generated on the common output waveform with respect to the output level of the segment output waveform are different.
In case {circle around (3)}, the changes in the segment output waveform are all directed in the same direction of the changes of the alternating signal, and for this reason the direction of the segment output waveform and the direction of the distortion generated on the common output waveform are the same, whereas in case {circle around (4)}, the changes in the segment output waveform are directed in the reverse direction of the changes of the alternating signal in the pixels on the segment electrodes X displaying a longitudinal OFF block display, and for this reason the direction of the segment output waveform and the direction of the distortion generated on the common output waveform are different, and the effective voltage of the distorted portion is increased as a result.
Therefore, the effective voltages applied to the liquid crystal cell 76 become different by the ratio of ON-OFF of display data when the alternating signal is changed and by the segment output voltage level. Namely, in case {circle around (3)} and case {circle around (4)}, VeffC less than VeffD, and as a result luminous non-uniformity called shadowing is generated.
For example, Japanese Unexamined Patent Publication No. 265402/1993 (Tokukaihei 5-265402) (published date: Oct. 15, 1993) discloses a technique for reducing luminous non-uniformity of display which is dependent on display pattern, in a liquid crystal display device of a simple matrix type which drives a liquid crystal panel in the described manner.
This technique is a driving method of a liquid crystal display device of a simple matrix type for driving a liquid crystal panel, for example, as shown in FIG. 17. Namely, a correction period is provided per line scanning period with respect to all outputs from a column driving circuit which corresponds to the segment side driving circuit 72. In the correction period, an intermediate voltage level of the ON display voltage level and OFF voltage display level is outputted as a correction voltage, instead of the display voltage outputted from the column driving circuit.
FIG. 17, FIG. 18, and FIG. 19 show common output voltage VY and segment output voltage VX applied to the liquid crystal panel in the technique disclosed in the above publication, in the case of a stripe display, whole screen ON display, and whole screen OFF display, respectively.
In the technique as disclosed in the above publication, the output waveforms of the segment side driving circuit 72 are all changed to an intermediate voltage level of an ON display voltage level and OFF display voltage level per single scanning period regardless of the display pattern. Thus, the number of output changes of the segment side driving circuit 72 becomes the same, thus reducing the display pattern dependent variation in effective value of the applied voltage. As a result, the above technique makes it possible to reduce luminous non-uniformity which is based on blunted waveform in accordance with the changes.
However, in the technique as disclosed in the above publication, an intermediate level is outputted in a correction period, and for this reason, compared with the effective voltage value applied to the liquid crystal cell 76 in a common driving method, the effective voltage value is reduced.
Thus, in the above technique, the contrast is lowered and the display quality suffers by the reduction in effective voltage value. Namely, as shown in FIG. 17, due to the fact that a portion 77 of the effective voltage at which an intermediate voltage level is applied to the segment electrodes X is lost, the effective value Veff of the voltage (VXxe2x88x92VY) applied to the liquid crystal cell is lowered, thus the problem that the display quality is susceptible to deterioration such as lowered contrast.
Japanese Unexamined Patent Publication No. 116056/1998 (Tokukaihei 10-116056) (published date: May 6, 1998), which is the invention of the inventors of the present application, proposes another technique for overcoming the above problem. FIG. 20 shows a common output voltage and segment output voltage applied to the liquid crystal cell of this publication.
In this technique, as the driving method of a liquid crystal display device of a simple matrix type, a display data signal and a display data signal of a preceding scanning period are independently compared per each pixel column, and when the display data signals are the same, a correction period whose period is fixed is provided in a single scanning period.
In this technique, in a correction period, an intermediate voltage level of an ON display voltage level and OFF display voltage level is outputted as a correction voltage, instead of a display voltage outputted from the segment side driving circuit 72.
Namely, when the display voltage level outputted from the segment side driving circuit 72 does not change for not less than two consecutive scanning periods, a voltage loss corresponding to blunted waveform generated when the display voltage level is changed to the ON display voltage level and OFF display voltage level in a correction period is corrected so as to reduce the display pattern dependent variation in the effective value of the applied voltage.
As a result, the above technique minimizes a voltage loss of the effective voltage applied to the liquid crystal cell 76, and solves luminous non-uniformity dependent on display pattern while minimizing lowering of contrast.
Another technique for solving the problem of variation in effective value of an applied voltage due to distortion of the common electrodes generated by a change in segment output waveform, which is another factor of shadowing, is disclosed in Japanese Unexamined Patent Publication No. 12030/1994 (Tokukaihei 6-12030) (published date: Jan. 21, 1994). In this technique, the waveform distortion of the common electrodes Y is detected, and by applying a reverse voltage of the distortion to the common electrodes Y, the variation in the effective value of the applied voltage due to waveform distortion of the common electrodes Y is reduced.
However, in the technique disclosed in the above publication, because the potential of the waveform distortion of the common electrodes Y is small, the mechanism for accurately detecting the waveform distortion becomes complex, and there arises the problem of increased cost due to the complicated mechanism.
The problems such as above are solved by the described technologies, yet the following problems remain to be solved. Namely, the technique disclosed in the above Japanese Unexamined Patent Publication No. 116056/1998 (Tokukaihei 10-116056) does not solve the problem of luminous non-uniformity due to distortion of the common electrodes Y, which is caused by a change in segment output waveform, which is another factor of shadowing. This problem cannot be solved unless this technique is combined with the technique disclosed in the above Japanese Unexamined Patent Publication No. 12030/1994 (Tokukaihei 6-12030). Thus, there remains the conventional problem that it is difficult to prevent deterioration of display quality and to simplify the mechanism at the same time.
An object of the present invention is to provide a liquid crystal display device and a driving method thereof which can improve display quality of a liquid crystal panel and simplify a system of the liquid crystal display device by eliminating a loss in effective voltage applied to a liquid crystal cell so as to solve luminous non-uniformity which depends on display pattern, and by correcting by a segment side driving circuit distortion of common electrodes generated by a change in segment output waveform.
In order to achieve the above object, a driving device of a liquid crystal display device of the present invention includes: a group of signal electrodes which are driven by a driving signal which corresponds to display data; a group of scanning electrodes which are disposed so as to intersect with the group of signal electrodes; a liquid crystal sandwitched between the group of signal electrodes and the group of scanning electrodes; an alternate control section for alternately and inversely driving the liquid crystal by an alternating signal; and a period control section for setting a changing period for changing an output level of the driving signal at least once in a single scanning period, and for controlling the changing period based on a change in the driving signal and a change in the alternating signal.
With this arrangement, a driving voltage error which is generated by a distortion generated on the group of scanning electrodes by a change in the driving signal of the group of signal electrodes can be corrected on the side of the driving signal of the group of signal electrodes by the control section, which controls the changing period for changing the output level, namely by adjusting the duration of the changing period.
Also, with the above arrangement, a driving voltage error which is generated by blunted waveform, etc., of the driving signal of the group of signal electrodes can also be corrected by comparing the driving signal with respect to the liquid crystal to be driven and the driving signal of the preceding scanning period based on whether they are at ON display level or OFF display level. Namely, when the driving signals compared are different, a voltage loss due to the driving voltage error by the blunted waveform is corrected on the side of the group of signal electrodes by controlling, that is, by adjusting by the control section the changing period for changing the output level.
In this manner, with the above arrangement, (a) the driving voltage error to be a loss of the effective voltage, due to blunted waveform of the driving signal of the group of signal electrodes, and (b) the driving voltage error generated by the distortion generated on the group of scanning electrodes by a change in waveform of the driving signal of the group of signal electrodes can be corrected by controlling the changing period for changing the output level, which is set in the driving signal, in a single scanning period based on a change in the driving signal and a change in the alternating signal, for example, by adjusting the duration of the changing period.
As a result, with the described arrangement, (a) the output waveform of a driving signal which is not to be corrected and (b) the output waveform of a driving signal which is to be corrected can be separately adjusted based on a change in the driving signal and a change in the alternating signal, thus making it possible to further reduce the difference on a display between the output waveform (a) and the output waveform (b). Thus, with the above arrangement, luminous non-uniformity on the display of the liquid crystal display device can be suppressed compared with the case where the period for changing the output level is fixed.
Further, with the above arrangement, the driving voltage error is corrected by predicting the generation of the driving voltage error based on a change in the driving signal and a change in the alternating signal. Thus, the conventional mechanism for detecting and correcting the driving voltage error can be omitted, thus simplifying the mechanism for correction.
As a result, with the above arrangement, by the provision of the control section which controls the changing period for changing the output level of the driving signal in a single scanning period based on a change in the driving signal and a change in the alternating signal, luminous non-uniformity on the display can be suppressed and the mechanism can be simplified.
In order to achieve the above object, another driving device of a liquid crystal display device of the present invention further includes a power source circuit for supplying (a) VC which is a non-select output level supplied to the group of scanning electrodes during a non-select period, and (b) VBH1, VBH2, VBL2, and VBL1, which are output levels of the driving signal, supplied to the group of signal electrodes, satisfying a relationship of VBH1 greater than VBH2 greater than VC greater than VBL2 greater than VBL1.
With this arrangement, the power source circuit supplies, as the output level of the driving signal, output levels of two values which are different from each other above and below VC, which is a non-select output level of the group of scanning electrodes.
Thus, driving of the liquid crystal in AC can be carried out by alternately outputting the output levels of two values between which is VC, and the changing period can be set by switching to the other output level on the side of the same polarity with respect to VC. This is realized by selecting the output level based on the alternate control section and the period control section.
It is also possible to set the output levels of two values on the side of the same polarity with respect to VC while placing between these two output levels a value corresponding to the output level of the conventional signal electrode group. Therefore, a decrease in effective voltage can be prevented even when the changing period is set.
As a result, with this arrangement, the changing period can be set by a circuit having a relatively simple structure, and it is possible to prevent lowering of contrast caused by a decrease in effective voltage, and deterioration of image quality even when the changing period is provided.
In order to achieve the above object, a driving method of a liquid crystal display device of the present invention for driving a liquid crystal sandwitched between a group of signal electrodes and a group of scanning electrodes which are disposed to intersect with the group of signal electrodes, by a driving potential difference between the group of signal electrodes and the group of scanning electrodes facing each other, includes the steps of: (1) providing in a driving signal of the group of signal electrodes a changing period for changing an output level of the driving signal at least once in a single scanning period; (2) controlling the changing period based on a change in the driving signal and a change in an alternating signal; and (3) correcting, by the control of the changing period, a driving voltage error generated by distortion which is generated on the group of scanning electrodes by the driving signal of the group of signal electrodes.
With this method, the driving voltage error which is generated by the distortion generated on the group of scanning electrodes by the driving signal of the group of signal electrodes are corrected by controlling the changing period for changing the output level of the driving signal of the group of signal electrodes in a single scanning period. Thus, compared with the conventional case where the changing period is fixed, luminous non-uniformity generated by the driving voltage error can be further reduced.
Further, with the above method, the driving voltage error is corrected by predicting the generation of the driving voltage error based on a change in the driving signal and a change in the alternating signal, and for this reason the conventional mechanism for detecting and correcting the driving voltage error can be omitted, thus simplifying the mechanism for correction.
As a result, with the above method, by controlling the changing period for changing the output level of the driving signal in a single scanning period based on a change in the driving signal and a change in the alternating signal, there are obtained effects of preventing luminous non-uniformity and simplifying the mechanism.
In order to achieve the above object, another driving method of a liquid crystal display device for driving a liquid crystal sandwitched between a group of signal electrodes which are driven by a driving signal in accordance with display data and a group of scanning electrodes which are disposed to intersect with the group of signal electrodes, includes the steps of: (1) comparing display data and display data of a preceding scanning period in a signal electrode group side driving circuit which drives the group of signal electrodes; and (2) correcting, when there is a difference between the display data compared in the step (1), a driving voltage error generated by blunted waveform of the driving signal, by controlling a changing period for changing an output level of the driving signal in a single scanning period.
The driving voltage error generated by blunted waveform of the driving signal is generated when there is a difference between display data and display data of a preceding scanning period. With the above method, the display data and the display data of the preceding scanning period are compared and when there is a difference between the two, the driving voltage error is corrected by controlling the changing period for changing the output level of the driving signal in a single scanning period. Thus, luminous non-uniformity generated by the driving voltage error can be further reduced compared with the conventional case where the changing period is fixed.
Further, with the above method, the driving voltage error is corrected by predicting the generation of the driving voltage error by comparing the display data and the display data of a preceding scanning period, thus omitting the conventional mechanism for detecting and correcting the driving voltage error and simplifying the mechanism for correction.
As a result, with the above method, by controlling the changing period for changing the output level of the driving signal in a single scanning period based on a change in the driving signal and a change in the alternating signal, there are obtained effects of preventing luminous non-uniformity on the display and simplifying the mechanism.
In order to achieve the above object, another driving method of a liquid crystal display device of the present invention for alternately and inversely driving by an alternating signal a liquid crystal sandwitched between a group of signal electrodes which are driven by driving signal in accordance with display data and a group of scanning electrodes which are disposed to intersect with the group of signal electrodes, includes the steps of: (1) comparing, when the alternating signal is inverted, display data and display data of a preceding scanning period in a signal electrode group side driving circuit which drives the group of signal electrodes; and (2) correcting, when the display data compared in the step (1) are the same, a driving voltage error generated by blunted waveform of the driving signal, by controlling a changing period for changing an output level of the driving signal in a single scanning period. The driving voltage error generated by blunted waveform of the driving signal is generated when the alternating signal is inverted and the display data and the display data of the preceding scanning period are the same. Thus, with the above method, the driving voltage error generated by such a blunted waveform of the driving signal is corrected in the signal electrode group side driving circuit by comparing the display data and the display data of the preceding scanning period, and when the display data compared are the same, by controlling the changing period for changing the output level of the driving signal in a single scanning period. Therefore, luminous non-uniformity generated by the driving voltage error can be further reduced compared with the conventional case where the changing period is fixed.
Further, with the above method, when the alternating signal is inverted, the driving voltage error can be corrected by predicting the generation of the driving voltage error by comparing the display data and the display data of the preceding scanning period, thus omitting the conventional mechanism of detecting and correcting the driving voltage error and simplifying the mechanism for correction.
As a result, with the above method, by controlling the changing period for changing the output level of the driving signal in a single scanning period based on a change in the driving signal and a change in the alternating signal, there are obtained the effects of preventing luminous non-uniformity on the display and simplifying the mechanism.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.